1. Technical Field
The present invention relates to a method of reducing the viscosity and molecular weight of a viscosified fluids and other polymer containing fluids used in subsurface hydraulic fracturing and stimulation operations, and more specifically relates to a method of reducing the viscosity and molecular weight of a viscosified fluids and other polymer containing fluids used in subsurface hydraulic fracturing and stimulation operations through the administration of a chelated transition metal.
2. Background Art
Hydraulic fracturing is a well-stimulation technique in which subsurface rock formations are fractured by the introduction of a hydraulically pressurized liquid. As a result of the cracks or fractures that are formed in rock formations, natural gas and/or petroleum flow and extraction from a well may be increased. Additional solid material additive components, commonly referred to as hydraulic fracturing proppants, may also be added to the hydraulic fracturing fluid to hold the fractures open.
Various form of hydraulically pressurized liquid are currently used, including viscosified fluids such as slick water, linear gels and cross-linked gels. While such viscous fluids are capable of carrying more concentrated proppant into subsurface rock formations, these viscofied fluids do present various drawbacks. One shortcoming regarding the use of viscosified fluids for stimulation and fracturing applications is the difficulty in effectively removing or extracting the viscofied fluid from the formation without removing the proppant from the fractures. Prior attempts to remove the viscosified hydraulic fracturing fluids have included the introduction of chemicals to reduce the viscosity of the fracturing fluids, commonly known as breakers. Examples of such break systems include the use of oxidative chemistries in various forms, as well as other methods that utilize chelants, surfactants, etc. However, many of these oxidative breaker systems do not achieve the desired degree of reduction in fluid viscosity and/or the desired degree of reduction in polymer molecular weight. Additionally, many of these prior oxidative breaker systems cannot perform the desired reduction in fluid viscosity and/or the desired reduction in polymer molecular weight in a delayed manner.
In order to increase the efficiency of oxidants used for reducing the viscosity of viscosified fluids, various catalytic agents have been employed for the purposes of decreasing reaction time and free radical generation.
The process of chelating transition metals has been shown to reduce the required concentration of transition metals, which in turn keeps certain transition metals in solution during their use as catalytic agents or redox agents which interact chemically with any number of bonds or linkages. Traditional chelating agents such as Sodium EDTA produces very stable, and therefore catalytically unavailable, complexes with transition metals. For example, the logarithmic stability constants, identified as log K in Table 1 below, for complexes of the chelating agent EDTA and certain transition metal complexes can be seen below in Table 1.
TABLE 1Metalionlog KCo3+41Fe3+25.1Hg2+21.8Cu2+18.8Ni2+18.6Pb2+18Cd2+16.5Zn2+16.5Co2+16.3Al3+16.1Fe2+14.3Mn2+13.8Ca2+10.6Mg2+8.7Ba2+7.9Ag+7.3
Despite these advancements and use of EDTA chelated transition metals as catalyzing agents in drilling related applications, it is still desirable to overcome present shortcomings to more efficiently reduce the viscosity of viscosified hydraulic fluids and/or reduce the molecular weight of the water soluble polymers within such fluids to desired levels.
Additionally, iron containing compounds have been shown to be effective catalysts in promoting oxidation-reduction (redox) reactions. In the classic Fenton reaction, identified below as Equation 1, a ferrous ion, Fe(II), rapidly reduces hydrogen peroxide to primarily hydroxyl radicals, which can react with and degrade a target contaminant. The reaction involves hydrogen peroxide and a ferrous iron catalyst. The peroxide is broken down into a hydroxide ion and a hydroxyl free radical. The hydroxyl free radical is the primary oxidizing species and can be used to oxidize and break apart organic molecules.H2O2+Fe2+→Fe3++HO−+HO*  (Equation 1)
In the classic Fenton reaction, a ferrous ion, Fe(II), is required in stoichiometric amounts. Peroxide demand, and therefore, ferrous ion demand can be high due to the required viscosity reduction. Ferrous ions also can be oxidized by the hydroxyl radicals, and therefore can compete with the target compounds unless its concentration is kept low by gradual addition in dilute form which may require undesirable or costly encapsulation in breaking applications. Accordingly, a method for introducing iron ions without substantial dilution or encapsulation is needed.
Ferric ion, Fe(III) can also produce hydroxyl radicals from peroxide, albeit at a slower rate than ferrous ion, Fe(II). The use of ferric ions, however, typically requires acidic conditions to keep the iron soluble, therefore, the classic Fenton reaction has an optimum pH of about 3. Such acidification, i.e., a low pH level, can cause undesirable issues with the performance of certain viscosifying agents and other fracturing fluid additives. Accordingly, a method for introducing ferric ions in a less acidic or non acidic environment is needed.